Influence of grain size on yielding asymmetry of extruded AZ31 magnesium alloy

Yielding asymmetry between tension and compression is typical in many wrought magnesium alloys. To clarify the influence of grain size on yielding asymmetry, AZ31 magnesium alloy with different grain size were prepared through extrusion followed by annealing treatment. Uniaxial tension and compression tests were carried out at room temperature to evaluate the yielding asymmetry of this alloy Optical microscopy and TEM were used to investigate the microstructure change just after yielding point. It is shown that, both tensile yield stress (TYS) and compress yield stress (CYS) decrease with increasing grain size. The influence of grain size on CYS/TYS ratio is non-monotonic and an extreme point appears at the grain size of 18.6 mu m. With the increase of grain size, deformation twinning contributes more to total strain. At a coarse grain size of 18.6 mu m, the contribution of twinning to total compression strain exceeds 50%, thus becoming a predominant deformation mechanism in this case.